1. Field of the Invention
This invention relates to methods of preparing imaged members and imaged members prepared thereby. In particular, but not exclusively, this invention relates to methods of preparing imaged printing forms, electronic parts and masks.
2. Background Information
Imaged articles, such as printing forms, electronic parts and masks, conventionally comprise a substrate onto which has been coated a film forming radiation-sensitive composition, the composition having been image-wise exposed to radiation of suitable wavelength, and developed to produce the imaged member.
A common form of printing plate used in the printing industry is the lithographic printing plate. Many lithographic printing plates are imaged within imagesetters. In the manufacture of such lithographic plates, plate precursors in the form of rolls or sheets of flexible material are generally fed into the image setting apparatus and digitally imaged within the imagesetter before being forwarded to prepress processing and then on to a printing press. Imagesetters generally include one or more rollers or angular components around which the flexible precursor must bend during imaging. Thus, the substrate of the precursor must be flexible enough to allow passage over rollers and angular components. As such, typical substrates used for lithographic printing forms include flexible polyester sheets and paper sheets. The inherent flexibility of these materials allows the precursor to travel round rollers and angular components with relatively little damage to the structure of the substrate and imaged precursor.
However, problems arise once the imaged precursor has travelled through the imagesetter and undergoes prepress processing and clamping to the press cylinder of the printing press. In order for efficient printing to be effected, the imaged member must be securely clamped to the printing press, and pulled taught such that there are no inconsistencies in the relief of the plate on the press. Generally, such plates are pulled taught by the practice of clamping both the leading and trailing edge of the plate to the print cylinder. The practice of clamping and tightening of the imaged member can easily stretch flexible substrates such as polyester and paper when mechanically stressed. Stretching of the substrate induces stretching of the imaged coating on the substrate, which may distort any image printed from that particular plate. Furthermore, there is a danger that, with particularly flexible substrates such as paper, tightening of the imaged member on the printing press will lead to tearing of the substrate with a subsequent loss of image.
Thus, the inherent flexibility of such plates while advantageous for the process of imaging in an imagesetter, also confers inherent dimensional instability on those substrates, which can be disadvantageous when mounting the substrate on a printing cylinder.
Other more dimensionally stable forms of substrate can be used, such as aluminum plates, but their inherent inflexibility precludes them from being passed through imagesetters. As imagesetters are used by many printing operatives around the globe, the cost of converting from using image setting equipment to equipment which can utilize inflexible aluminum plates can be financially prohibitive.
Other imaged members such as flexographic printing plates and printed circuit boards are commonly made from thick sheets of semi-flexible plastic substrate. The thickness of the sheet is used to effect sufficient dimensional stability to the substrate against stresses encountered during use. The use of thick substrates is relatively expensive and there is a desire in the industry to reduce substrate thickness whilst maintaining dimensional stability.
For flexographic plates in particular, historically these imaged members have been imaged by using film as a masking medium. The need for separate masking medium is relatively labor intensive and enhances the cost of producing such flexograhpic plates. The flexographic printing plate industry has consequently been looking for ways to reduce costs and labor intensity of producing such plates. One method of reducing costs and labor intensity, would be to adopt the digital imaging readily available in image setting equipment, which eliminates the need for masking medium and its associated costs. However, the thickness and relative inflexibility of the substrates used in flexographic printing, compared to the flexible substrates used in lithographic printing, prevents their use in conventional filmsetting equipment.
EP 644064 (Agfa) discloses a lithographic printing plate comprising on a first side of a flexible support a surface differentiated in oleophilic and oleophobic areas and on the second side a layer of micro particles of pressure sensitive adhesive. The layer of pressure sensitive adhesive is covered by a covering layer, which layer is peeled off before the flexible support is adhered to a printing press surface under tension. The micro particles do not increase the dimensional stability of the flexible support and merely serve to allow adhesion of the printing form precursor on the printing press.
U.S. Pat. No. 4,092,925 (Fromson) discloses a lithographic printing plate system comprising an aluminum printing member having a light sensitive coating thereon applied by a carrier plate releasably adhered to the aluminum printing member. The carrier plate increases the dimensionally stability of the aluminum printing member, but the inflexibility of the aluminum printing member prevents the aluminum printing member being passed through an image setter prior to adhesion of carrier plate.
U.S. Pat. No. 2,048,964 (Osborn) discloses a planar graphic printing plate comprising a non-metallic core surface encased on both sides by metallic facings, the metallic facings being coextensive with the core and secured thereto to waterproof the site thereof. The planar graphic printing plate is non-flexible, and cannot be passed through an imagesetter containing multiple roller and sharp angles. Furthermore, the lamination of two sides of metallic facings is relatively expensive.
EP 690349 (Dupont) discloses a flexible lithographic printing form comprising a laminate of flexible substrate and aluminum sheeting. The resultant laminate is flexible enough to be passed through an imagesetter, but the use of an aluminum sheeting is relatively expensive, and special laminating equipment and procedures must be employed in order to create the laminated plates.
U.S. Pat. No. 4,032,684 (Dunnington et al) discloses a method of manufacturing a composite lithographic printing form, the printing form comprising a metal surface having a lithographic quality composition coated thereon, and a backing sheet laminated to the uncoated surface of the metal sheet. As described for EP 690349 A, the printing forms disclosed in U.S. Pat. No. 4,032,684, being laminated, are relatively expensive to manufacture and involve the use of complicated laminating machinery and laminating procedures.
WO 93/10979 (Aloisi) discloses a pre-sensitized lithographic printing plate comprising a backing film of plastic material laminated to a sheet of aluminum foil, which is coated with a photosensitive composition, The thickness of the backing material is 1 to 4 times greater than the thickness of aluminum foil. As with EP 690349 A, the printing forms disclosed in WO 93/10979 are laminates, requiring specialised laminating machinery and laminating procedures, including the use of adhesives. The need for multiple layers of metal and backing increases production costs considerably as compared to a single layers of lithographic printing form.
U.S. Pat. No. 3,979,212 (Peters et al. ) discloses a lithographic printing form comprising an aluminum sheet laminated to a non-stainless steel sheet with an adhesive. The laminate disclosed in U.S. Pat. No. 3,979,212 is inflexible and cannot be used through an imagesetter which comprises rollers and sharp angles. Furthermore, the use of multiple metallic laminated sheets is relatively expensive and requires specialized machinery for manufacture.
EP 807534 A (Agfa) discloses a flexible supported lithographic printing plate having improved dimensional stability. The printing plate of EP 807534A comprises a flexible support to which is laminated a dimensionally stable base suitably adapted for mounting on a printing press. The dimensionally stable bases are disclosed to be aluminum plates. As described for the previously disclosed prior art, the laminated plates of EP 807534 A are relatively expensive to manufacture due to the need for multiple layers and laminating equipment and procedure.
WO 98/53371 discloses an image plate comprised of a at least one stiffening layer formed from a first polymer material and a print contact layer formed from a second polymer material laminated together. The print contact layer is a relief contact layer having raised protrusions which form the print image of the printing plate. The print contact layer is covered by a releasable layer until an image is desired to be printed. The printing form of WO 98/53371 is a letterpress printing form, which cannot be passed through lithographic imagesetters. imagesetters. Furthermore, the laminated structure of the letterpress printing forms of WO 98/53371 are relatively expensive produced due to the need for multiple layers, laminating equipment and procedures.
JP 3073392 A (Mitsubishi Papermills Limited) discloses a base material for a lithographic printing plate, the base material comprising a resin coated paper base material, the base material being impregnated with a composition hardenable by electron beam radiation. The resin coating is disclosed as being a fused polyolefin resin layer which is also electron beam hardenable. The printing plate is formed by irradiating the coated paper such that the impregnated paper and resin coating are both hardened by electron beam radiation. The resultant printing plate can then be coated with a lithographic imaging composition and imagewise exposed and developed. In this printing plate, the printing plate is first dimensional stabilized and then passed through an imagesetter for imagewise exposure. The use of a double layer of hardenable material in which both layers are electron beam irradiated is relatively expensive and the addition of an imageable coating increases the thickness of the dimensionally stabilized imagesetter handling capabilities of the substrate.
There is therefore a need in the lithographic printing, flexographic printing and printed circuit board industries for a substrate which is both imagesetter compatible in its flexibility, but which after imaging is dimensionally stable enough to endure the mechanical stress of being tightened over a printing cylinder, or on printing surface in the case of circuit boards, and which is relatively cheap and quick to manufacture.
Traditionally four printing units are employed in succession for four-color printing; yellow, cyan, magenta and black units in series. Each unit carries a printing plate. If flexible plates are used, a further problem can arise due to distortion of one or more flexible plate of the four units, resulting in color overlap between two or more colors, leading to image distortion and unacceptable print images. For this reason plastics and paper substrates have generally been restricted to black and white, and spot color (black and white with one color) printing.
It is therefore an aim of preferred embodiments of the present invention to overcome or mitigate at least one of the above problems of the prior art, or other problems, whether expressly described hereinabove or not.